I have read multiple post regarding a question of more power but my questions are a little different. I have a 1975 Roadrunner with 39000 original miles. It has a 360 with a stock intake and a 600 cfm thermoquad. (I think) it is a TQ though. 3.21 stock gears with a 340 stock automatic cam. It has dual exhaust but I would like to get more 0-30 performance. The car is strong after 30+ but as it being a very heavy car it doesn't have much off the mark performance. So I have thought about different rear end gear but keeping some sanity on highway speeds. It has also the stock heads and so wondering what all of you would do? I have read about different torque converters but I am a little confused on that subject. The car is original and it is only street driven. I know everyone here has a lot knowledge and experience so open to suggestions. Thanks everyone.
360 in a 75 Roadrunner
- Thread starter drodg58
- Start date
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LocuMob
Fluid Technician
You don't see many 75 Roadrunners. Put 3.55's or 3.73's to give it some oomph. But then you might need to go to a higher stall convertor, I'm not to learned in that area either. Just don't so anything you can't undo, that's a low mileage example. A friend has one with the 400 in it. Was going to sell.it but backed out of the deal, and I'm glad he kept it.
d1970
TODAY IS A GOOD DAY TO HAVE A GOOD DAY
you can join For B Bodys Only website for an answer to your question to, seeing it's a 75 Roadrunner.
yellow rose
Overnight Sensation
I agree. Even though it's a 75 that thing is pretty cool. A 3.55-3.73 gear would help.
TrailBeast
AKA Mopars4us on Youtube
More rear gear or more torque are your choices for any reasonable change.
You can get more torque with another converter.
This lets the engine get into it's best RPM range for producing torque on takeoff.
Displacement is another solution, but obviously this takes a lot more money, a bigger motor or both.
I would have to say the biggest change for the least amount of cash is going to be rear gears and the converter would be next.
I have been on the B bodies only sight for years and have pictures posted there. I have read multiple times for the best advice for a small block is here so I joined. Thanks for the advice on the gearing. So what type of TC would you go with or what stall speed? Thanks again
LocuMob
Fluid Technician
A fine compliment to the small block crowd here.
TrailBeast
AKA Mopars4us on Youtube
That would be best suggested by the convertor manufacturer.
Call them up and tell them what you have and what you want out of it.
Be specific and as accurate as you can on the answers to the questions they ask.
LocuMob
Fluid Technician
Is the hood factory paint?
It is I believe the V21 hood blackout. I bought the car about five years ago. It is mostly original paint though I had the black resprayed as it was a tad rough looking. But it is original to the car. It is a nice car. They drive like Cordobas, so actually quite nice.
ken5124
65 Valiant Signet
Nice car!! I had an 81 Cordoba from new, for ten years. rode great.. Wish I still had it!
72Valiant4Door
Trouble Maker
Gears
mikedevore
Well-Known Member
I had a Chrysler 5th Avenue a few years back with a E58 spec 360.
Car weighed 4100lbs.
It had a 2.94 rear gear in it.
I used a low gear 904 transmission (2:74 vs 2:45 low gear).
stock convertor.
It did have headers & fake cats on it.
But still had all the comfort items.
It ran 14.80's all day & had plenty of low end to break the tires loose.
Had a '73 T-Quad on it.
I would bet that your car has a 727 transmission in it. There is a 2:77 low gear available for it, but it is pricey.
The nice thing about the low gear in the transmission, it lets you maintain the cruising speeds that you have now.
Car weighed 4100lbs.
It had a 2.94 rear gear in it.
I used a low gear 904 transmission (2:74 vs 2:45 low gear).
stock convertor.
It did have headers & fake cats on it.
But still had all the comfort items.
It ran 14.80's all day & had plenty of low end to break the tires loose.
Had a '73 T-Quad on it.
I would bet that your car has a 727 transmission in it. There is a 2:77 low gear available for it, but it is pricey.
The nice thing about the low gear in the transmission, it lets you maintain the cruising speeds that you have now.
LocuMob
Fluid Technician
That's cool, I've never seen that before, but I have seen less than a dozen in person.
67Dart273
Well-Known Member
There was one of those Cordoba / RR's for sale around here awhile back. I have no idea if it had the "Corinthian leather"
I did not used to like those bodies, but I guess I'm getting old. If I had the money I'd have one. TURBO THAT THING!!
Turboed 62 Savoy
My Turbocharged '62 Savoy 4-door
I did not used to like those bodies, but I guess I'm getting old. If I had the money I'd have one. TURBO THAT THING!!
Turboed 62 Savoy
My Turbocharged '62 Savoy 4-door
72Valiant4Door
Trouble Maker
I put a low first gear set in my 904 and LOVE IT.
ir3333
Well-Known Member
3.21 / 3.23 are great gears and i'd leave your 3.21's alone unless you go to a 27 or 28" tire.
.. a call to one of the converter manufacturers will get you what you want.
.. a call to one of the converter manufacturers will get you what you want.
DartGTDan
'71 Dart GT Fan
Years ago my sister had a '75 RR. It was gold, had a black RR decal on the trunk lid. Didn't realize how rare it was. I should have bought it from her. Live and learn.
67Dart273
Well-Known Member
I can still remember my 70RR........3.54 and G60X15 results in 3000 @ 70mph. JUST RIGHT!!!
(Actually Wallace says 66.6 but that was before Al invented the internet!!)
I agree with, leave the rear end alone.
This seems like a low buck hope for big improvement. There are some things you can do that are better bang for your buck but you're starting with 4100 lbs and 180 hp.
Changing gear ratio in trans isn't cheap, nethier is changing the rear end, unless you put in an overdrive, I wouldn't change the gearing.
Headers, cam, valve springs, new timing chain. Cam that is big in th torque department.
This seems like a low buck hope for big improvement. There are some things you can do that are better bang for your buck but you're starting with 4100 lbs and 180 hp.
Changing gear ratio in trans isn't cheap, nethier is changing the rear end, unless you put in an overdrive, I wouldn't change the gearing.
Headers, cam, valve springs, new timing chain. Cam that is big in th torque department.
nm9stheham
Well-Known Member
Your'e not looking for more power..... your'e looking for more low RPM torque. Your cam is the problem. That cam was made for the 340 when it had a true compression ratio of over 9:1. The main reasons to put a longer duration, slow ramp, moderate lift cam into the 340 was to stay with the lower lift for valvetrain reason, and the long duration at low lift made sure that the thing ran without detonation on bad gas and poor maintenance.
Switch to a different cam. Different gears are not not going to fix the low RPM dead spot, just cover it up to a degree and move the speed at which the car really rakes off to a lower speed like 25 mph instead of 30 mph. The higher stall converter will just let the engine rev up past the low torque RPM band when taking off, but it will lower fuel economy. (Dunno if that is important to you.) If you are drag racing this thing, then gear changes and higher stall TC changes make all the sense in the world. But for pure street driving, they will be just patches to cover for the wrong cam.
Your 360 has a true compression ratio of about 7.6-7.8. Put in a long advertised duration cam like that 340 cam in a higher compression 340 and it'll get past the low RPM's without too much trouble. Now, put that same cam into a low compression 360 and the DCR and the low RPM torque is just gonna go into the toilet. Take some cranking compression readings on all cylinders and I bet you will find them down in the 110 psi range, which is lower than a stock 360. So there goes your low RPM torque....
Look into a 256 or 262 advertised duration cam, with a faster cam ramp. Crane H-260-2, Voodoo 10200701 or 100200702, Racer Brown EH-8, or XE 256 or 262 are good candidates. All will have about the same or even a bit higher lift than the 340 auto cam that you now have, and the durations will be a lot shorter to boost up the DCR and the low end torque.
Right now, your DCR is down in the 6.2 range. That's bad juju for low RPM torque. As a point of reference for where your engine is right now, a really torquey small block will have a DCR in the high 7's range or better, and cranking compression numbers in the 140-150 psi range or better.
Change to a Voodoo 10200701 and install it with an ICL of 104 and now DCR is up to 6.8. That's getting better and your 360's longer stroke will help to work better with that DCR. The cranking compression numbers will be up around 125 psi now, not fantastic, but at least a bit better than stock. And these new cams will still breath at the mid and high RPM ranges like the 340 cam.
Work on the ignition timing after that to get it more advanced. That will help the low-mid RPM torque even more. (And if you want to do better yet for low RPM torque, you can mess with heads and head gaskets to get higher DCR and even better torque. But that is another level of work.)
Once done with the cam change to get past that glaring problem, and optimizing ignition timing, you can re-evaluate for the different gears and TC stall speeds.
Switch to a different cam. Different gears are not not going to fix the low RPM dead spot, just cover it up to a degree and move the speed at which the car really rakes off to a lower speed like 25 mph instead of 30 mph. The higher stall converter will just let the engine rev up past the low torque RPM band when taking off, but it will lower fuel economy. (Dunno if that is important to you.) If you are drag racing this thing, then gear changes and higher stall TC changes make all the sense in the world. But for pure street driving, they will be just patches to cover for the wrong cam.
Your 360 has a true compression ratio of about 7.6-7.8. Put in a long advertised duration cam like that 340 cam in a higher compression 340 and it'll get past the low RPM's without too much trouble. Now, put that same cam into a low compression 360 and the DCR and the low RPM torque is just gonna go into the toilet. Take some cranking compression readings on all cylinders and I bet you will find them down in the 110 psi range, which is lower than a stock 360. So there goes your low RPM torque....
Look into a 256 or 262 advertised duration cam, with a faster cam ramp. Crane H-260-2, Voodoo 10200701 or 100200702, Racer Brown EH-8, or XE 256 or 262 are good candidates. All will have about the same or even a bit higher lift than the 340 auto cam that you now have, and the durations will be a lot shorter to boost up the DCR and the low end torque.
Right now, your DCR is down in the 6.2 range. That's bad juju for low RPM torque. As a point of reference for where your engine is right now, a really torquey small block will have a DCR in the high 7's range or better, and cranking compression numbers in the 140-150 psi range or better.
Change to a Voodoo 10200701 and install it with an ICL of 104 and now DCR is up to 6.8. That's getting better and your 360's longer stroke will help to work better with that DCR. The cranking compression numbers will be up around 125 psi now, not fantastic, but at least a bit better than stock. And these new cams will still breath at the mid and high RPM ranges like the 340 cam.
Work on the ignition timing after that to get it more advanced. That will help the low-mid RPM torque even more. (And if you want to do better yet for low RPM torque, you can mess with heads and head gaskets to get higher DCR and even better torque. But that is another level of work.)
Once done with the cam change to get past that glaring problem, and optimizing ignition timing, you can re-evaluate for the different gears and TC stall speeds.
Once again thanks for the advice. I am probably considering the rear axle ratio change as it is the least cost for the time being. I think down the road when I want to change the cam I will probably do a head change also to increase the compression ratio. You guys have been very insightful and a great help.
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